Investment Securities, AFS and HTM Securities Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Dec. 31, 2019
Investment securities available for sale
Amortized Cost	$ 2,897,851	$ 3,251,950
Gross Unrealized Gains	54,231	30,920
Gross Unrealized (Losses)	(23,295)	(20,284)
Fair Value	2,928,787	3,262,586
Investment securities held to maturity
Held to Maturity, Amortized Cost	2,149,434	2,205,083
Held-to-maturity, Gross Unrealized Gains	100,534	79,744
Held-to-maturity, Gross Unrealized Loss	(2,414)	(8,363)
Held-to-maturity, Fair Value	2,247,553	2,276,465
Debt Securities, AFS, Principal Sold	281,000
U. S. Treasury securities
Investment securities held to maturity
Held to Maturity, Amortized Cost	999	999
Held-to-maturity, Gross Unrealized Gains	42	19
Held-to-maturity, Gross Unrealized Loss	0	0
Held-to-maturity, Fair Value	1,041	1,018
U.S. government SBA agency securities
Investment securities available for sale
Amortized Cost	10,241
Gross Unrealized Gains	2
Gross Unrealized (Losses)	(5)
Fair Value	10,238
Investment securities held to maturity
Held to Maturity, Amortized Cost	0
Held-to-maturity, Fair Value	0
Obligations of state and political subdivisions (municipal securities)
Investment securities available for sale
Amortized Cost	492,195	529,908
Gross Unrealized Gains	16,443	16,269
Gross Unrealized (Losses)	(1)	(18)
Fair Value	508,636	546,160
Investment securities held to maturity
Held to Maturity, Amortized Cost	1,426,543	1,418,569
Held-to-maturity, Gross Unrealized Gains	82,382	69,775
Held-to-maturity, Gross Unrealized Loss	(475)	(1,118)
Held-to-maturity, Fair Value	1,508,451	1,487,227
FFELP asset backed securities
Investment securities available for sale
Amortized Cost	360,658	270,178
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(19,234)	(6,485)
Fair Value	341,424	263,693
Investment securities held to maturity
Held to Maturity, Amortized Cost	0
Held-to-maturity, Fair Value	0
Other debt securities
Investment securities available for sale
Amortized Cost	3,000	3,000
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	0	0
Fair Value	3,000	3,000
FNMA / FHLMC \| Residential mortgage-related securities
Investment securities available for sale
Amortized Cost	103,576	131,158
Gross Unrealized Gains	908	1,562
Gross Unrealized (Losses)	0	(59)
Fair Value	104,485	132,660
Investment securities held to maturity
Held to Maturity, Amortized Cost	78,125	81,676
Held-to-maturity, Gross Unrealized Gains	3,486	1,759
Held-to-maturity, Gross Unrealized Loss	0	(15)
Held-to-maturity, Fair Value	81,610	83,420
FNMA / FHLMC \| Commercial mortgage-related securities
Investment securities available for sale
Amortized Cost	19,861	19,929
Gross Unrealized Gains	2,912	1,799
Gross Unrealized (Losses)	0	0
Fair Value	22,773	21,728
Investment securities held to maturity
Held to Maturity, Amortized Cost	0
Held-to-maturity, Fair Value	0
GNMA \| Residential mortgage-related securities
Investment securities available for sale
Amortized Cost	799,983	982,941
Gross Unrealized Gains	23,804	3,887
Gross Unrealized (Losses)	0	(1,689)
Fair Value	823,787	985,139
Investment securities held to maturity
Held to Maturity, Amortized Cost	245,675	269,523
Held-to-maturity, Gross Unrealized Gains	8,124	1,882
Held-to-maturity, Gross Unrealized Loss	0	(1,108)
Held-to-maturity, Fair Value	253,799	270,296
GNMA \| Commercial mortgage-related securities
Investment securities available for sale
Amortized Cost	1,108,336	1,314,836
Gross Unrealized Gains	10,162	7,403
Gross Unrealized (Losses)	(4,055)	(12,032)
Fair Value	1,114,443	1,310,207
Investment securities held to maturity
Held to Maturity, Amortized Cost	398,092	434,317
Held-to-maturity, Gross Unrealized Gains	6,500	6,308
Held-to-maturity, Gross Unrealized Loss	(1,939)	(6,122)
Held-to-maturity, Fair Value	$ 402,653	$ 434,503

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